1. Technical Field
The present invention relates to drive systems for driving field winding synchronous machines that are used in, for example, motor vehicles as electric motors, electric generators and motor-generators.
2. Description of Related Art
Conventionally, there have been known various synchronous machines used in motor vehicles. These synchronous machines generally include a stator and a rotor. The stator includes a stator core and a stator coil wound on the stator core. The stator coil is comprised of a plurality of phase windings that together create a rotating magnetic field. The rotor is rotatably disposed so as to radially face the stator through an electromagnetic gap formed therebetween.
For example, Japanese Patent Application Publication No. JP2015015846A discloses a field winding synchronous machine that includes a rotor. The rotor includes a rotor core having a plurality of main poles and a plurality of auxiliary poles formed therein, a plurality of main field windings that are wound respectively on the main poles of the rotor core and connected in series with each other, and a plurality of excitation windings that are wound respectively on the auxiliary poles of the rotor core and connected in series with each other. In the field winding synchronous machine, torque is generated by exciting the main field windings by causing excitation magnetic flux to directly cross the main poles of the rotor core.
However, in the field winding synchronous machine disclosed in the above patent document, the main field windings of the rotor are excited by time harmonics superimposed on electric current supplied to the stator coil and harmonic components of the stator magnetic field caused by a magnetic circuit; and torque is generated by the field current and the stator current. Consequently, torque ripple is caused by the harmonic components.